1. Field of the Invention
The present invention relates to an output circuit of a vacuum-tube amplifier, and more particularly, to an output circuit of a vacuum-tube amplifier, which maintains a voltage between a ground terminal and an output terminal of the vacuum-tube amplifier as 0V to output an amplified signal without using a coupling condenser or a transformer and maintains a voltage of a cathode of a vacuum tube of an output stage uniform by a variable self-bias circuit to obtain linearity of output power in proportion to an increase in an input signal without being affected by the cathode when the input signal is increased, increase the output power and improve distortion.
2. Background of the Related Art
In spite of advantages of a semiconductor device, lovers of music still prefer an audio amplifier using a vacuum tube because the audio amplifier using a vacuum tube has sound quality higher than that of a semiconductor amplifier.
FIG. 1 is a circuit diagram of an output circuit of a conventional vacuum-tube amplifier. Referring to FIG. 1, the output circuit of the vacuum-tube amplifier includes an amplification unit and an output buffer unit. The amplification unit receives an input signal through a grid G of a vacuum tube 1, receives a voltage V1 through a plate P of the vacuum tube 1, amplifies the input signal, and outputs the amplified signal through a cathode K of the vacuum tube 1. A coupling condenser C1 and a bias resistor R1 are connected to the cathode K of the vacuum tube 1 such that the amplified signal is output to an output node N1 through the coupling condenser C1.
In case of a multi-stage amplifier, multiple vacuum tubes having the same configuration as the vacuum tube 1 are connected in parallel, a bias resistor and a coupling condenser are connected to the cathode of each of the multiple vacuum tubes, and output nodes are commonly connected in parallel. FIG. 1 illustrates only one vacuum tube.
The output buffer unit receives the signal of the output node N1 of the amplification unit through a grid G1 of a vacuum tube 2 and outputs the amplified signal to a speaker 4 through a plate P1 of the vacuum tube 2. A resistor R2 and an AC bypass condenser C2 are connected in parallel with a cathode K1 of the vacuum tube 2 to construct a self-bias circuit 5.
To output a signal amplified by the vacuum tube 1 in the amplification unit, the coupling condenser C1 or a transformer for cutting off a DC voltage must be used at the cathode K of the vacuum tube 1. A general method sets the resistance of the resistor R1 such that a voltage, obtained by halving the plate voltage V1 of the vacuum tube 1, is set to the cathode K of the vacuum tube 1. That is, when the amplification unit is directly connected to a following amplifier, the coupling condenser C1 or a transformer must be used in order to cut off a DC signal and pass only an AC signal because there is a DC components in a signal transmitted to the following amplifier.
The output buffer unit connects the resistor R2 of hundreds to thousands ohm to the cathode K1 of the vacuum tube 2. A bias voltage is applied to the vacuum tube 2 using a voltage drop according to the resistance R2. That is, a self-bias method is used in the output buffer unit.
FIG. 2 illustrates cathode and output power characteristics with respect to an input signal of a self-bias amplification circuit. In the self-bias circuit as illustrated in FIG. 1, when the voltage applied to the grid G1 is increased, current of the cathode K1 is increased in proportion to the increase in the voltage applied to the grid G1. Here, the voltage is increased in proportion to the current by resistance set to the cathode K1. A voltage according to a basic current value of the cathode K1 and the input signal of the grid G1 are increased in proportion to the input signal voltage in a satisfactory manner in response to the amplification factor of the vacuum tube 2. When an input signal having a voltage exceeding the voltage according to the basic current value of the cathode K1 and the input signal voltage of the grid G1 is applied, however, an increase in the voltage of the cathode K1 attenuates the input voltage of the grid G1 by the increase in the voltage of the cathode K1. Accordingly, the input signal and output power do not achieve satisfactory linearity, as illustrated in FIG. 2.